Everything You Need to Know About Heartworm Disease

An adult heartworm removed from a dog.An adult heartworm removed from a dog.
An adult heartworm removed from a dog.An adult heartworm removed from a dog.

Table of Contents:

  1. What Is Heartworm Infection?
  2. Who Gets Infected With Heartworm?
  3. How Is Heartworm Spread?
  4. How Is Heartworm Disease Prevented?
  5. How Is Heartworm Treated?
  6. What Happens If Heartworm Is Untreated?

Spring has sprung! Along with the April showers, we welcome National Heartworm Awareness Month with this review of heartworm and ways to safeguard your pets.

What Is Heartworm Infection?

An infected heart containing parasitic worms. 

 

Many parasites can infect our furry family members, so it is common to think that heartworm is just like the other worms that take up residence in the gastrointestinal tract. This is not the case. True to its name, the heartworm is a worm that lives in the heart and vessels of the heart. The heart plays an important role in providing blood throughout the body, making blockages caused by worms especially dangerous, since they can compromise blood-pumping ability.

Who Gets Infected With Heartworm?

Dogs, cats, and even ferrets can be infected by heartworm, however, progression to heartworm disease occurs most commonly in dogs than cats. Treating heartworm disease is a complex process, therefore, much focus is around prevention of infection. Because mosquitoes play an important role in transmitting infection, pets that live in warmer/humid parts of the country are more at risk, as these areas are prime breeding ground for mosquitoes. That being said, heartworm has been found in each of the 50 states and in Canada.

Did you know?

Wild animals can be infected with heartworm. Examples include: coyotes, wolves, foxes, California sea lions, and raccoons.

How Is Heartworm Spread?

The transmission of heartworm is dependent on a vector: the female mosquito. When the mosquito feeds on the blood of an infected dog, an immature version of the heartworm (known as microfilaria) is ingested with this meal as well. This immature stage then matures within the mosquito into a larval stage. It is at this point that the heartworm larvae are ready to infect, entering an animal host through a tiny hole made as the mosquito feeds. Over a period of several months, this larval stage travels via the host bloodstream to the pulmonary vessels and heart. At this stage, the larva is now an immature worm that will mature into an adult heartworm. The worm will reproduce and release the next batch of microfilaria to be ingested by a feeding mosquito. Left untreated, the cycle will repeat itself.

How Is Heartworm Disease Prevented?

Heartworm is commonly prevented with monthly oral or topical medications or 6 month injectable medications that can only be prescribed or administered by a licensed veterinarian. It is a misconception that monthly medication works by preventing or blocking infection for the month ahead. Conversely, preventives work by killing the younger versions that have already infected the pet in the past 30+ days, which makes it more like a retroactive treatment. In addition to preventing heartworm, many of these medications also contain components that act as dewormers, flea/tick prevention, and mite treatment.

Heartworms in a petri dish.

Did you know?

When cats are infected with heartworm, the worm burden is usually only 1 to 3 adult worms. However, their presence can lead to a syndrome called Heartworm Associated Respiratory Disease syndrome or HARD.

How Is Heartworm Treated?

If a dog tests positive for heartworm, the first thing to do is to repeat the test to confirm the diagnosis as well as check the blood for microfilaria. Next, chest radiographs (x-rays) are taken to evaluate the heart and lungs to see if the heartworms have created any side effects such as cardiac enlargement (+/- failure) and/or vessel enlargement. Treatment will depend on how far along the disease has progressed.

Generally treatment consists of 3 stages:

  1. Pre-treatment. Treatment with doxycycline to treat a secondary parasite that infects at the same time as the heartworm.
  2. Injection #1. A deep intramuscular injection of melarsomine (an organic arsenic compound), plus a prescription for a steroid to decrease the body’s immune response in reaction to dying heartworms and their components.
  3. Injections #2 & #3. One month after the first injection, repeat intramuscular injections given 24h apart and repeat steroid treatment.

Treating heartworm disease is a complex and costly process, requiring much care from the veterinarian and even more from the pet parent. Dog parents have the added responsibility of keeping their pet as calm and quiet as possible to prevent adverse events resulting from dying worms.

What Happens If Heartworm Is Untreated?

If left untreated, adult worms will likely reproduce, adding further burden to the heart and its vessels, as well as adding to the propagation of heartworm in the area via microfilaria in the bloodstream. In rare cases, where there is only one worm burden, it will not be able to multiply, but may be able to cause damage as it dies.

It is far easier, cheaper, and safer to protect your pet from heartworm disease with prevention. The saying “An ounce of prevention is worth a pound of cure.” definitely holds true in this case.

Have a safe Heartworm Awareness Month!

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